1. Field of the Invention
The present invention relates to: a method of processing digital data; apparatus for processing digital data; a computer program for processing digital data; a data format; a method and apparatus for restoring the processed data; and a corresponding program. Some aspects and embodiments of the invention relate to processing audio signals, but it will be appreciated that in other aspects and embodiments the invention may be applied to other data. The other data may be audio/visual data, video data, still image data, a computer program, and multimedia data amongst other examples.
2. Description of the Prior Art
EP-A-1,189,372 (Matsushita) discloses many techniques for protecting audio signals from misuse. In one technique, audio is compressed and encrypted before distribution to a user. The user needs a decryption key to access the audio. The key may be purchased by the user to access the audio. The audio cannot be sampled by a user until they have purchased the key. Other techniques embed an audible watermark in an audio signal to protect it. In one technique, an audio signal is combined with an audible signal (also referred to as a watermark) according to a predetermined rule. The watermark degrades the audio signal. The combination is compressed for transmission to a player. The player can decompress and reproduce the degraded audio signal allowing a user to determine whether they wish to buy a “key” which allows them to remove the watermark. The watermark is removed by adding to the decompressed degraded audio signal an equal and opposite audible signal. The watermark may be any signal which degrades the audio. The watermark may be noise. The watermark may be an announcement such as “This music is for sample playback”.
Co-pending UK patent application 0202737.3 (Sony United Kingdom Limited) filed 6 Feb. 2002 discloses a method of modifying a compressed video bitstream for applying a watermark to the video. The bitstream includes digital codes representing the compressed video. At least one digital code is selected. The code occupies a part of the bitstream which is to contain at least one watermark code which represents a watermark perceptible in the information signal. The selected, digital code(s) are removed from the said part of the bitstream. The watermark code(s) are put in the said part of the bitstream in place of the selected code(s). The number of bits of the selected code(s) removed from the said part of the bitstream is greater than or equal to the number of bits of the said watermark code(s) put in the said part. The removed selected code(s) are encrypted and appended to an end of the bitstream and/or placed in watermark user data fields created in the bitstream.
WO 02/51150 discloses a system in which an audio signal is transmitted in the blanking period of a video signal. Compressed video information and compressed audio information are reproduced from a DVD. The video is decoded by a computer. The computer encrypts the decoded video. The computer also encrypts the compressed audio. The computer outputs the encrypted video and the encrypted audio via a cable.
U.S. Pat. No. 6,041,160 combines, in a multiplexer, encoded audio information with compressed video which has been at least partially scrambled.
US-A1-2002/0108043 generates MPEG encoded data which comprises audio and video information. A switch has two inputs: one is connected to receive the MPEG encoded data directly and the other via an encrypting device. The switch selects one or the other input to generate partly encrypted MPEG data.
U.S. Pat. No. 4,266,243 comprises a mixer which combines composite video signals with scrambled and compressed audio signals. The audio signals are scrambled in a scrambling device and then the scrambled audio is compressed in a compressor before being applied to the mixer.
GB-A-2 369 022 (Radioscape Limited) describes the delivery of audio files by digital radio,
An incomplete and/or partly corrupted audio file in compressed form, is transmitted as a first data stream by digital radio “in the clear” (i.e. unencrypted). An audio file of n frames is made incomplete by removing n-n frames, leaving m to be transmitted. The file is corrupted by corrupting m-p of those frames leaving p uncorrupted frames to be transmitted “in the clear”.
A second data stream referred to as a “delta stream” comprising the n-m removed frames and the m-p original (uncorrupted) frames totalling n-p frames. Those n-p frames are encrypted. In one example those n-p encrypted frames are embedded within extra space allocated within the audio frames themselves.
The receiver is able to reassemble the original audio file by reinserting the n-m removed frames from the second stream into the first data stream and replacing the m-p corrupted frames in the first data stream with the m-p original frames taken from the second stream.